Context improvements

Remove currentAccount and stuff from XmppConnectivity. Save last used
account ID in vars.context. App no longer calls XmppConnectivity.client,
currentAccount, currentAccountName etc. - which are no longer required.
Add a new menu for MUC (ConversationPage will choose a proper menu for
chat, muc and archive anyway), currently not functional at all.  Fix
subscriptions (they were broken apparently).